Mercury is both a global pollutant and a contaminant that can be transformed to a potentially toxic species (e.g., methylmercury) under natural conditions. Mercury emitted to the atmosphere can travel thousands of miles before being deposited to the earth. Studies show that mercury from the atmosphere can also be deposited in areas near the emission source. Mercury intake by human beings, especially children, can cause a variety of health problems.
Coal-fired power plants and medical and municipal waste incineration are major sources of human activity relating to mercury emission to the atmosphere. It is estimated that there are 48 tons of mercury emitted from coal-fired power plants in US annually. However, so far there is no effective mercury emission control technology available at a reasonable cost, especially for elemental mercury emission control.
The state of the art technology that has shown promise for controlling elemental mercury as well as oxidized mercury is active carbon injection (ACI). The ACI process includes injecting active carbon powder into the flue gas stream and using fabric fiber (FF) or electrostatic precipitator (ESP) to collect the active carbon powder that has adsorbed mercury. Generally, ACI technologies require a high carbon to Hg ratio to achieve the desired mercury removal level (>90%), which results in a high cost for sorbent material. The high carbon to Hg ratio suggests that ACI does not utilize the mercury sorption capacity of carbon powder efficiently. A major problem associated with ACI technology is cost. If only one particle collection system is used, the commercial value of fly ash is sacrificed due to its mixing with contaminated activated carbon powder. A system with two separate powder collectors and injecting activated carbon sorbent between the first collector for fly ash and the second collector, or a baghouse, for activated carbon powder, may be used. Baghouse with high collection efficiency may be installed in the power plant facilities. However, these measures are costly and may be impractical, especially for small power plants.
Since water-soluble (oxidized) mercury is the main mercury species in bituminous coal flue gas with high concentrations of SO2 and HCl, bituminous coal-fired plants may be able to remove 90% mercury using a wet scrubber combined with NOx and/or SO2 control technologies. Mercury emission control can also achieve as a co-benefit of particulate emission control. Chelating agent may be added to a wet scrubber to sequestrate the mercury from emitting again. However, a chelating agent adds to the cost due to the problems of corrosion of the metal scrubber equipment and treatment of chelating solution. However, elemental mercury is the dominant mercury species in the flue gas of sub-bituminous coal or lignite coal and a wet scrubber is not effective for removal of elemental mercury unless additional chemicals are added to the system. The prior art discloses adding various chemicals to the gas stream to aid the removal of mercury. However, it is undesirable to add additional potentially environmentally hazardous material into the flue gas system.
Certain industrial gases, such as the syngas produced in coal gasification, may contain toxic elements such as arsenic, cadmium and selenium, in addition to mercury. It is highly desired that all these toxic elements be substantially abated before the syngas is supplied for industrial and/or residential use.
There is a genuine need of a sorbent material capable of removing mercury and/or other toxic elements from fluid streams such as flue gas and syngas with a higher capacity than activated carbon powder alone. It is desired that such sorbent material can be produced at a reasonable cost and conveniently used, such as in a fixed bed.
The present invention satisfies this need.